Conventional magnetic resonance (MR) pulse sequences include a preparation phase, a waiting phase, and an acquisition phase that serially produce signals from which images can be made serially. The preparation phase determines when a signal can be acquired and determines the properties of the acquired signal. For example, a first pulse sequence may be designed to produce a T1-weighted signal at a first echo time (TE) while a second pulse sequence may be designed to produce a T2-weighted signal at a second TE. These conventional pulse sequences are typically designed to provide qualitative results where data are acquired with various weightings or contrasts that highlight a particular parameter (e.g., T1 relaxation, T2 relaxation).
When MR images are generated, they may be viewed by a radiologist and/or surgeon who interprets the qualitative images for specific disease signatures. The radiologist may examine multiple image types (e.g., T1-weighted, T2-weighted) acquired in multiple imaging planes to make a diagnosis. The radiologist or other individual examining the qualitative images may need particular skill to be able to assess changes from session to session, from machine to machine, and from machine configuration to machine configuration. Thus, the images are only as good as the image interpreter and all image based (e.g., qualitative) diagnoses end up being subjective.
Magnetic resonance fingerprinting (MRF) employs a series of varied sequence blocks that simultaneously produce different signal evolutions in different resonant species (e.g., tissues) to which the RF is applied. The term “resonant species”, as used herein, refers to an item (e.g., water, fat, tissue, material) that can be made to resonate using NMR. By way of illustration, when example apparatus and methods apply RF energy to a volume that has both bone and muscle tissue, then both the bone and muscle tissue will produce an NMR signal. However the “bone signal” and the “muscle signal” will be different. The different signals can be collected over a period of time to identify a signal evolution for the volume. Resonant species in the volume can then be characterized by comparing the signal evolution to known evolutions. In one embodiment, the “known” evolutions may be, for example, simulated evolutions and/or previously acquired evolutions. Conventional MRF may store a large set of known evolutions in a dictionary. Characterizing the resonant species can include identifying different properties of a resonant species (e.g., T1, T2, diffusion resonant frequency, diffusion co-efficient, spin density, proton density). Additionally, other properties including, but not limited to, tissue types, materials, and super-position of attributes (e.g., T1, T2) can be identified.
MRF is described in United States Patent Application “Nuclear Magnetic Resonance (NMR) Fingerprinting”, application Ser. No. 13/051,044, and in Magnetic Resonance Fingerprinting, Griswold et al., Nature 495, 187-192 (14 Mar. 2013), the contents of both of which are incorporated herein by reference.